Conventionally, a motor drive device PWM-modulates a DC voltage into AC by the switching of switching elements of an inverter circuit, outputs the voltage to a motor coil of three phases, and thereby drives a position-sensorless three-phase motor. In such a motor drive device, it is known that a modulation scheme for PWM modulation is selectively switched between a three-phase modulation scheme and another modulation scheme of fixing the on/off states of switching elements of at least one phase.
For example, in a motor control device disclosed in the following Patent Literature 1, a 180-degree sine wave drive scheme is applied during motor start-up and in a low-speed rotation region, and a 120-degree conduction rectangular wave drive scheme is applied in a high-speed rotation region.